The invention relates to the repeated use of an adhesive-film laminate for redetachable bonds, said redetachment taking place by pulling on the laminate in the direction of the bond plane, and to adhesive-film laminates suitable for this purpose.
Adhesive films for redetachable bonds which can be redetached by pulling in the direction of the bond plane are known and are commercially available under the designation xe2x80x9ctesa Power-Stripsxe2x80x9d. Bonds produced therewith offer a powerful hold and yet can be redetached without a trace and without damaging the substrate or the bonded parts, as is described in DE 33 31 016 C2. Moreover, DE 42 33 872 C2 describes adhesive films of this kind together with a hook, the hook itself being completely reusable. Despite the many undisputed advantages of these products, they have the disadvantage that the adhesive films cannot be used again and thus repeatedly. Given the considerable price of these adhesive films, this is a significant disadvantage for the user.
Nor from other prior art is it possible to find any such products which can be used again, and thus repeatedly.
WO 92/11333 and WO 93/01979 describe redetachable adhesive tapes with an intermediate support comprising an extensible but not resilient film. Such films, although they can be removed again by pulling in the direction of the bond plane, are in no case reusable, since these products, after xe2x80x9cstrippingxe2x80x9d, are unable to relax back into their original state. Admittedly, WO 92/11332 describes redetachable adhesive tapes which also use extensible, resilient films as intermediate supports, but exclusively photopolymerized pressure-sensitive acrylate adhesives are used, which give such products disadvantages whose consequences in practice are decisively disruptive. Reproducible crosslinking of the pressure-sensitive adhesive composition is difficult to attain, resulting in corresponding fluctuations in the product properties. Moreover, a residual content of photo-initiator is unavoidable, which, especially in the case of subsequent bonding under the action of sunlight, for example on window panes, leads to distinct changes in the layer of adhesive; to after crosslinking, yellowing and film formation, so that stripping without residue is no longer possible. In addition, the unavoidable content of residual monomer (at least 1%) is objectionable on health grounds, especially for interior applications. Similarly, follow-on products of the photoinitiator, especially methyl benzoate, may lead to migration and alteration of the product properties. The heat of reaction which is liberated during two-dimensional UV polymerization of acrylates may damage or corrugate the support. Compounding with, for example, resins is possible only to a very limited extent, since these resins disrupt UV polymerization. Also, the inevitably obtained crosslinking profile of the adhesive composition is a further disadvantage: normally, UV polymerization is carried out by UV irradiation on the composition side, leading to layers of adhesive with greater crosslinking on the surface of the adhesive than on the support. The result is reduced tack and poor anchorage of the composition. If UV irradiation is carried out through the support, which brings about better tack and improved adhesion and anchorage of the composition, the support must be UV-permeable. However, many supports and many SBS/SIS block copolymers do not possess particularly good UV permeability, or are damaged by UV light.
Overall, the product properties mean that prolonged bonding in constant quality is not ensured, and stripping without residue is in particular not possible, as demonstrated in WO 92/11332 on p. 19, Table 2, column 6, where residues of the composition remain at the edges of the bond (footnote a), if indeed the products do not tear (footnote b). Products which can be used more than once are therefore neither described nor suggested.
Furthermore, U.S. Pat. No. 4,024,312 describes highly extensible adhesive films which have a support consisting of highly elastic, thermoplastically processable styrene block copolymers of type A-B-A (where A=poly(-styrene), B=poly(isoprene), poly(butadiene) or hydrogenation products thereof). Resins which can be mixed with the block polystyrene domains can be added, optionally, to the support, preferably in a quantity of from 85 to 200 parts per 100 parts of elastomer. The support is coated on at least one side with a pressure-sensitive adhesive composition. The elastic deformation of the adhesive tapes is at least 200%, and the modulus at 50% extension is  less than 13.8 MPa (2000 lbs/inch2). Elastic reformation after stretching by 50% is at least 75%. The pressure-sensitive adhesive compositions employ either polyisoprene (e.g. natural rubber) or the synthetic rubbers which are also employed for the support material and are based on styrene block polymers, blended with adhesive resins and, if desired, with further blend components. Adhesive tapes can be removed readily from the substrate by stretching parallel to the bond surface. As a result of the migration capacity of the low molecular mass constituents (resins, plasticizers) between pressure-sensitive adhesive composition and support, such self-adhesive tapes are not constant in their product properties: the mechanical strengths of the support and properties of the pressure-sensitive adhesive compositions used are irreversibly altered by the diffusion of the resins. Controlled setting and regulation of the product properties, as is essential for industrial bonds, is therefore not possible. Repeated use is impossible since the support materials coated with rubber adhesive, when stretched parallel to the bond surface, and immediately after detachment from the substrate, shoot back and stick together on the adhesive side with such force that these rubber coatings cannot be detached from one another. Products which can be used more than once are therefore neither described nor suggested.
The object of the invention was to remedy this situation and, in particular, to enable an adhesive-film laminate to be used more than once.
The invention relates accordingly to the repeated use of an adhesive-film laminate, as characterized in more detail in the claims.
Elastic supports are, in particular, elastomers with a resilience of  greater than 50%, preferably  greater than 80%; during the process of detaching the bonded products, extensions of from 100 to 400% occur on average. Values of up to 1500% may also arise. In this context, the extension which occurs is essentially a function of the tensile expansion behaviour of the type of elastomer employed, of the thickness of the support and of the bond strength which is attained by the adhesive films.
In every case, a prerequisite for residue-free redetachment from the substrate or from the bonded joint is that the tear strength of the adhesive film must be higher than its stripping force. The ratio of tearing force to stripping force is preferably  greater than 1.5, particularly preferably  greater than 2.5.
The tear strength of the adhesive films described here is determined in particular by the nature and thickness of the support films used. For supports based on styrene block copolymers with a linear three-block structure and block polystyrene contents of from about 15 to 40% by weight, the minimum support thickness for double-sided pressure-sensitive adhesive films, with adhesive forces of  greater than  about 5 N/cm, is about 50 xcexcm (see examples). Below this limit, the tear strength is normally too low for redetachment without residue. For substrates with a high degree of adhesion relative to the adhesive films employed (high adhesion forces), the minimum support thickness required is increased correspondingly.
Preferred elastomers are:
1. Styrene Block Copolymers.
Suitable such copolymers are styrene-isoprene and styrene-butadiene block copolymers and their hydro-genation products, styrene-ethylene/butylene and styrene-ethylene/propylene block copolymers. Block copolymers according to the invention can be linear SES (S denotes the polystyrene block, E the elastomer block) three-block polymers, or else radial and star-shaped (SE)x block copolymers (x denotes the n-functional coupling component) where nxe2x89xa73, and linear (SE)n block polymers.
Typical block polystyrene contents are in the range from about 8 to 50% by weight, preferably between about 15 and 45% by weight. The SE two-block content to be chosen is preferably  less than 50%.
2. Natural rubber
3. Polyisoprene
4. Polybutadiene
5. Polychloroprene rubber
6. Butyl rubber
7. Silicone rubber
8.. EPDM rubber or ethylene-propylene copolymers
9. Polyurethanes (e.g. Walopur 2201/Wolff Walsrode, Platilon UO 1/Atochem, Desmopan/Bayer, Elastollan/Elastogran)
10. Vinyl copolymers
10a. Ethylene-vinyl acetate copolymers (e.g. From M and W: 524.060; from Exxon: Exxtraflex Film)
10b. Vinyl chloride-acrylate copolymers
11. Polyether esters (e.g. Arnitel/Akzo, Hytrel/Du Pont)
12. Polyetheramides and polyesteramides (e.g. Pebax/ Atochem, Grilon/Ems-Chemie)
13. Polycarbonate-polyester copolymers
14. Ethylene-acrylate copolymers
15. ABS copolymers.
The abovementioned elastomers can also be employed as a constituent in polymer blends.
To establish the mechanical properties, crosslinking of the abovementioned materials may be advantageous.
Suitable pressure-sensitive acrylate adhesives, including blend components (adhesive resins, fillers, pigments) are:
solvent-containing and solvent-free acrylate adhesive compositions:
Copolymers based on acrylic acid/methacrylic acid and esters thereof with 1 to 25 carbon atoms, maleic acid, fumaric acid, itaconic acid and their esters, substituted (meth)acrylamides; further vinyl compounds, for example vinyl esters, vinyl acetate, vinyl alcohol and/or esters thereof
Compounds of acrylate copolymers and resins, for example Foral 85 E
Compounds of various acrylate copolymers
Compounds of acrylate copolymers and further polymeric blend components
Optionally, additives in the form of inorganic and organic materials can be used in accordance with the invention, examples being glass balls, glass fibres, pigments, anti-ageing agents, carbon black and titanium dioxide.
In order to produce sufficient cohesion, the acrylate copolymers used are normally crosslinked. To achieve uniform crosslinking density across the thickness of the layer, thermally initiated crosslinking methods are appropriate, for example crosslinking by way of metal chelates. A very homogeneous crosslinking profile can also be achieved by means of irradiation with electron beams. The control parameter for the crosslinking density profile is the accelerator voltage of the electron-beam source. Depending on the weight per unit area of the adhesive film to be irradiated, it is possible in this case to choose one-sided irradiation (preferably in the case of low weights per unit area, but also in the case of high weights per unit area if a sufficiently high accelerator voltage is available) or electron-beam irradiation on both sides (preferred in case of high weights per unit area and low accelerator voltages) in order to establish a homogeneous crosslinking density.
As a result of their high molar mass, dispersion acrylates usually have a level of cohesion which is sufficient for the applications described herein, so that no additional crosslinking is generally required.
In order to improve the anchorage of the pressure-sensitive adhesive compositions on the intermediate support, the latter can be given a physical and/or chemical pretreatment (priming). Examples of suitable pretreatment methods are corona pretreatment, flame pretreatment, plasma pretreatment and pretreatment with fluorine.
The products used in accordance with the invention exhibit a range of advantages over the prior art.
Relative to natural rubber- and synthetic rubber-based pressure-sensitive adhesives with olefinic double bonds (DE-33 31 016, U.S. Pat. No. 4,024,312 etc.):
the high ageing resistance, i.e. in particular no loss or rise in the parameters of finger tack, adhesive force and shear strength after ageing, even after severe exposure to sunlight or ozone;
very high transparency coupled with very low inherent colour: systems of water-clear transparency can be produced.
Relative to three-layer laminates with synthetic-rubber pressure-sensitive adhesive based on styrene block polymers (U.S. Pat. No. 4,024,312, etc.):
in the absence of low molecular mass constituents of the adhesive composition, there is no danger of its migration between the laminate layers of the adhesive films and into the surface of the bonded items.
Relative to single-layer laminates and to systems with nonelastic intermediate supports (DE 33 31 016, WO 92/11333, etc.):
separate control of adhesion and tear strength, since the adhesion can be controlled primarily by way of the outer adhesive-tape layers and the tear strength via the internal, middle layer.
Broad capacity for adjusting the stripping forces in the case of residue-free detachment, as a result of the high variability in the moduli of elasticity of the elastic middle layers (support films) which are available.
Relative to UV polymers/UV-crosslinked pressure-sensitive adhesive compositions (WO 92/11332):
no danger of aftercrosslinking, especially after exposure to sunlight, owing to residual content of UV initiators. No residual content of toxicologically objectionable constituents as are commonly employed in UV-polymerized or, respectively, crosslinked systems.
Ease of preparation of pressure-sensitive adhesive compositions of specific coloration.
Possibility of incorporating fillers into the pressure-sensitive adhesive compositions.
Possibility of using mixtures of already polymerized copolymers.
In general:
Ease of preparation in the production process, since can be carried out with the customary coating techniques.
Elastic intermediate supports with elastic reformation of preferably  greater than  about 90% can be used repeatedly when sufficiently ageing-resistant pressure-sensitive adhesive compositions are used, since the layer thickness, which primarily determines the product quality (finger tack, adhesive force, shear strength) goes back on extensive reformation to its original extent. In contrast to rubber-based pressure-sensitive adhesive compositions, the auto-adhesive properties of acrylate copolymers can easily be adjusted such that the adhesives are easy to remove from one another even after contact.
Moreover, the acrylate compositions employed in accordance with the invention, since they can be applied from solution, the melt or dispersion, can be anchored outstandingly to viscoelastic supports. In addition, the compositions do not have the crosslinking profile typical of UV compositions. The crosslinking density is uniform throughout the adhesive layer. This particularly good anchorage of the products is demonstrated by adhesion tests on steel and by failed attempts at delamination. Unlike UV-polymerized products, the products leave no residues of adhesive composition on substrates such as steel, and the anchorage can be improved even further by pretreating the support (corona, flame pretreatment, plasma, primer).
In the text below the intention is to illustrate the invention on the basis of working examples without wishing thereby unnecessarily to restrict the invention.